Ad hoc networks are communication networks devoid of fixed infrastructure. A certain number of wireless stations are equipped with radio emission and/or reception means and with appropriate protocols to form the nodes of the ad hoc network.
These stations making up the ad hoc network can be in the form of fixed or portable computers, pocket computers, mobile telephones, vehicles, electrodomestic appliances, etc. The emission-reception means can also be associated with simple objects such as sensors or actuators. An ad hoc network of sensors thus makes it possible to perform information collection for example with a view to monitoring or controlling installations.
The success of ad hoc networks depends a great deal on the lifetime of the stations constituting the nodes of the network. Energy saving is a crucial factor for designing long-lifetime sensor networks, in particular because on the one hand the nodes are generally powered by cells which are generally expensive and difficult, or even impossible, to replace or recharge and because on the other hand, energy sources such as cells or batteries are sources of pollution.
Ad hoc networks have severe operating constraints. First of all, there is no fixed infrastructure. The nodes have to cooperate to allow exchanges. Additionally, the structure of the network varies: new nodes can integrate the network, while nodes disappear, in particular when they no longer have any energy reserve. When data has to be dispatched from one node to a final recipient node, the data are transferred via several successive hops between intermediate nodes, each intermediate node being within radio range of the previous node and of the following node.
To allow in particular each node, liable to emit data, to address these data to a node which on the one hand lies within radio range of the emitter node and on the other hand is the most appropriate to be the next intermediate node to receive and emit the data, each node has information on any node situated within radio range. The nodes situated within radio range of a given node constitute the 1-hop neighbourhood of the given node, which varies as a function of several factors such as the introduction or the disappearance of nodes, the distance between the given node and the neighbouring nodes, the transmission powers of the nodes, the gains and radiation patterns of the antennas respectively associated with the nodes, the interference etc.
This information available to each node X, relating to the nodes of its 1-hop neighbourhood, is in particular utilized to define, within the framework of routing decisions, the node Y, from among those constituting the 1-hop neighbourhood of node X, to which X has to address a message to be forwarded ultimately to a target node D.
This information defining the 1-hop neighbourhood is obtained in the prior art with the aid of periodic exchanges of information. Each node periodically dispatches a data packet dubbed “HELLO” containing its identifier and a value associated with this node, dubbed the metric. By listening to the HELLO packets thus emitted, each node X constructs and maintains a data table comprising the identifiers of the nodes from which node X has recently received a HELLO packet and their respective metric value. This table defines the nodes making up the 1-hop neighbourhood of node X. When a node X has to select a node from among the nodes of its 1-hop neighbourhood, to which to transmit data, it makes this selection as a function of the data table available to it.
This technique nevertheless presents drawbacks. First of all, the dispatching of the HELLO packets allowing the updating of a node's information table takes place only at given periods. The information contained in the table is therefore rapidly obsolete and the decisions taken pursuant to the utilization of this information are therefore not always apt.
Additionally, the implementation of these systematic exchanges consumes a not-inconsiderable quantity of energy, independently of whether or not a node uses the information contained in a new HELLO packet received.
Additionally, this mechanism for exchanging information and for utilizing the table can be viewed as belonging to the routing layer and does not take into account the current constraints at the level of the MAC layer, which leads to energy losses.
There is therefore a need to gather, at the level of a node, non-obsolete information characterizing the nodes of its 1-hop neighbourhood, and to limit the energy expenditure of the nodes.